The rate at which a human heart beats is controlled by a feedback loop provided by a neurohumoral mechanism, the basic component of which is the autonomic nervous system, i.e. the sympathetic and parasympathetic nervous system. Heart rate variability is often decreased in severe ischaemic heart disease, congestive heart failure, ageing and diabetic neuropathy. Decreased HRV has been used as a tool in the early identification to of patients with severe ischaemic heart disease and severe heart failure who are at high risk of sudden death.
The electrocardiograph (ECG) signal is indicative of electrical currents in the heart muscle. The ECG signal of a single heart beat can be divided into several components. The P wave represents the spread of an impulse through the atria just before atrial contraction. This is followed by a QRS complex reflecting the spread of an impulse through the ventricles just before they contract. Currents generated as the ventricles recover appear in the ECG as a T wave. The time between consecutive R peaks, known as the RR interval, is normally used as a basis for heart rate measurements because the R peaks are relatively easy to detect.
The Poincaré plot is a scatter plot of current RR interval plotted against the previous RR interval, i.e. the ith RR interval in the series, RRi, is plotted as a function of the previous RR interval of the series, RRi-1. The Poincaré plot thus consists of points of the form (RRi, RRi-1). The Poincaré is based on the observation that the length of a heartbeat is significantly determined by the length of the previous heart beat. When the variation in RR intervals over time is small, the plot will consist primarily of a relatively dense cluster of points. When the variation in the RR intervals is significant, the points of the plot will be scattered. The Poincaré plot thus provides a graphical representation of the RR data, which facilitates evaluation of HRV.
The Poincaré plot has been generalized to the so-called “m-lagged Poincaré plots” in which RR; is plotted as a function of RRi-m, where m is an integer that may be greater than 1. It has been observed that the length of a heartbeat can affect several subsequent heartbeats.
U.S. Pat. No. 6,731,974 to Levitan et al discloses measuring heart rate variability by assigning a unit mass to each point in a Poincaré plot, and calculating the product of the quadrupole moments of the two axes of the plot.
U.S. Pat. No. 6,532,382 to Meier et al discloses calculating heart rate variability from an ECG signal by measuring discrete measuring values representative of the heart rate variability, and evaluating the Fourier transform of the measuring values.